1. Field of the Invention
The present invention relates to a gate drive circuit of a wide band gap semiconductor such as GaN, SiC and the like, and more particularly, to a technology enabling optimal driving of a GaNFET.
2. Description of the Related Art
A power conversion apparatus, which uses switching of a semiconductor switch for power to convert alternating current (AC) power or direct current (DC) power to AC power or DC power having a different level, is used in a UPS apparatus, an inverter for a motor, a DC-DC converter and the like.
The semiconductor switch for power has an on-resistance between a drain and a source and has been improved to a substantial theoretical value in an existing silicon device. However, in recent inverters and DC-DC converters, a novel low on-resistance device is needed so as to further improve efficiency.
Therefore, it is expected to commercialize a SiC or GaN device having potential exceeding related-art silicon devices.
Regarding this, from the related-art silicon device, a wide band gap semiconductor such as GaN, SiC and the like, i.e., a normally-off type GaNFET has been developed. A recent normally-off type FET has been developed to have a threshold voltage of 1 voltage to several voltages.
FIGS. 3A to 3C shows respective static characteristics of the normally-off type GaNFET, in which FIG. 3C shows a characteristic of a gate/source voltage versus a gate current. Regarding the characteristics of the normally-off type FET, as shown in FIG. 3C, a structure between a gate and a source is not an insulating structure such as related-art silicon MOSFET. Thus, when an excessive gate drive voltage is applied, large current flows between the gate and the source and a characteristic similar to a forward voltage of a diode is thus exhibited. Due to this, when a voltage larger than is needed is applied to the gate, power consumption of a gate drive is increased, compared to the related-art silicon MOSFET, or when current more than is needed flows in the gate, damage is caused, so that problems are caused with respect to reliability.
FIG. 10 is a circuit diagram showing an example of a gate drive circuit of a related-art silicon MOSFET. When a gate pulse signal is input, a voltage between a gate and a source is faster increased by a time constant circuit of a speedup capacitor C1 and a resistance R2, which is connected in parallel with a gate resistance R1 (refer to JP-A-10-163838).
FIG. 11 is a circuit diagram showing an example of a base drive circuit of a related-art bipolar transistor. In order to realize a high-speed switching of the bipolar transistor Qa, a method has been known in which sufficient over drive current is enabled to flow through a capacitor C1 to turn on the bipolar transistor Qa, the excessive base current is shunted to a collector by diodes D1, D2 so that the bipolar transistor Qa becomes an on-state of a non-saturated voltage, and thus accumulated charges between a base and an emitter of the bipolar transistor Qa due to the excessive base current are thus reduced to shorten a storage time period of a switching characteristic (refer to JP-A-61-101119).
However, in a semiconductor switch comprised of a high electron mobility transistor (HEMT) using the wide band gap semiconductor such as GaN, which has the characteristics of FIGS. 3A to 3C, pressure-resistance of the gate to the over-voltage is low. In addition, the characteristics between the gate and the source are similar to the forward voltage characteristics of the diode. Further, unlike the related-art silicon MOSFET, when a gate pulse voltage of about 10V is applied, the gate current is enabled to flow, so that the reliability is lowered and the gate drive voltage is increased.
When the non-saturated drive is performed as the drive circuit of the bipolar transistor, the low loss during on-time due to the low temperature resistance characteristic, which is a characteristic of the GaNFET, cannot be utilized.